Whichever of the dominant two clubs in England over the last decade takes the 2011 Premier League title, there will be gripes.
Manchester United, even though they might seize a domestic and European double, find themselves lightly disdained when their current team is contrasted with more dazzling United squads. Chelsea suffer likewise, with frailties detected in areas where the editions of 2005 and 2006 was formidable.
One shared source of critical comparison is centre of midfield. Carlo Ancelotti's Chelsea, unlike Jose Mourinho's version, have no Claude Makelele, so reliable at regaining possession that people still talk about "the Makelele role".
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Sir Alex Ferguson's up-to-date United have won an impressive array of titles since their former captain, Roy Keane, left the club, but they have not done so with any midfield player as regularly dynamic, aggressive or thrusting.
Though Ferguson has been reassured lately to see Darren Fletcher's energies returning, satisfied that Michael Carrick has regularly imposed himself and smiled at Anderson's relish for killing off a Champions League semi-final, none are clones of Keane.
United have been through a list of central midfield players in the last six years. Mikel John Obi, who will anchor Chelsea's midfield today, was earmarked as potentially one of the best. The Nigerian had been scheduled to join United from the Norwegian club, Lyn, two months after Keane's departure in November 2005.
The transfer never happened because of a long, ugly and expensive custody battle between United and Chelsea over who had rights to the then-teenaged Obi.
Chelsea ended up paying United around £12 million (Dh72m) in compensation for a player whose natural role, in nearly 150 Premier League matches, has yet to be fully defined. For Nigeria, where he plays as an advanced, attacking midfielder, Mikel is a standout.
"He's got everything," said Daniel Amokachi, the former Nigeria striker, who coached Mikel as a teenager, "and it is a strength that he can play anywhere in midfield."
The recent signs are that Mikel is finally making the "Makelele role" - deep, protecting the central defenders - a comfortable home.
It has been long journey. John Obi was born in the northern Nigerian city of Jos 24 years ago. His older brother became a professional goalkeeper, but by his mid-teens, now using the name "Mikel", Obi junior was the family's star. Outstanding performances at the 2003 Under 17 World Championship in Finland brought him attention.
Lyn's coup in signing the starlet turned into a burden as the question of which English club they had sold him on to went all the way to an Oslo court, amid allegations of threats and forged signatures.
The effect on the young Mikel of the controversy was hard to gauge. Carlos Queiroz, then the United assistant coach, said a tearful kid had been effectively "kidnapped" by Chelsea. Others describe a tough young man.
"He's not a quiet lad, he's sociable," said Joseph Yobo, his international teammate, "and when he had those problems with the contracts, he just focused on his football." That focus has, at times, strayed. His Chelsea career is punctuated by several red cards, punctuality problems and a conviction for drunken driving.
But there are recent signs of maturity on the field, that his great athletic potential, based around height, strength, poise and accurate delivery of a pass, especially from set-pieces, might genuinely be best exploited as a midfield general. Ancelotti may be nurturing in Mikel some of what he cultivated at Milan with Andrea Pirlo, who also converted his game from front-of-the-midfield-diamond to base-of-midfield.
Mikel was man of the match when Chelsea's win over Tottenham Hotspur revived their title prospects. His club need him to produce more of the same up against the likes of Carrick. Or Fletcher. Or Anderson.
MATCH-UP
Key Battle
Nemanja Vidic v Didier Drogba Forget Fernando Torres, Drogba is the man United will fear most. If he is at the top of his game, Chelsea's Ivorian striker has the strength and pace to terrorise and cause United's backline severe problems. Vidic, below, was sent off at Stamford Bridge in March and, as good as he is, can struggle against Drogba's brute force.
Tactics
The situation plays into Chelsea's hands as they have to win to take the upper hand in the title race. United know they can afford a draw, but it is rarely Sir Alex Ferguson's style to play for a point, especially at home where their record is formidable.
Previous meetings
All eyes point to last season's "title-decider" at Old Trafford, which Chelsea won convincingly, but recent history points to the Champions League tie barely a month ago, when United were convincing victors.
Probable line-ups
Man Utd (4-5-1) Van der Sar; Fabio, Vidic, Ferdinand, Evra; Carrick, Park, Giggs, Nani, Valencia; Rooney
Chelsea (4-3-3) Cech; Ivanovic, Terry, Luiz, Cole; Mikel, Essien, Lampard; Malouda, Drogba, Torres
Facts
• Chelsea were the last visiting team to win at Old Trafford, in April 2010.
• Chelsea have one clean sheet in their last 15 trips to Old Trafford.
